^{115}In^{+}-^{172}Yb^{+} Coulomb Crystal Clock with 2.5×10^{-18} Systematic Uncertainty

We present a scalable mixed-species Coulomb crystal clock based on the ^{1}S_{0}↔^{3}P_{0} transition in ^{115}In^{+}. ^{172}Yb^{+} ions are cotrapped and used for sympathetic cooling. Reproducible interrogation conditions for mixed-species Coulomb crystals are ensured by a conditional preparation sequence with permutation control. We demonstrate clock operation with a 1In^{+}-3Yb^{+} crystal, achieving a relative systematic uncertainty of 2.5×10^{-18} and a relative frequency instability of 1.6×10^{-15}/sqrt[τ/1  s]. We report on absolute frequency measurements with an uncertainty of 1.3×10^{-16} and optical frequency comparisons with clocks based on ^{171}Yb^{+} (E3) and ^{87}Sr. With a fractional uncertainty of 4.4×10^{-18}, the former is-to our knowledge-the most accurate frequency ratio value reported to date. For the ^{115}In^{+}/^{87}Sr ratio, we improve upon the best previous measurement by more than an order of magnitude. We also demonstrate operation with four ^{115}In^{+} clock ions, which reduces the instability to 9.2×10^{-16}/sqrt[τ/1  s].